1. Field of the Invention
This invention relates to a magnetron, more particularly to an improvement in its cathode support structure.
2. Background of the Prior Art
In the cathode support structure of the conventional magnetron for use in microwave ovens, as illustrated in FIG. 17, the two ends of the filament cathode 21 which is coiled are fixed to a pair of end caps 22, 23, either directly or via a guide 24. A pair of cathode support rods 25, 26, made of molybdenum, are fixed to the two end caps 22, 23. These cathode support rods 25, 26 pass to the outside via throughholes 28 which run right through the ceramic stem 27, and are bonded hermetically to terminal strips 29, which are brazed hermetically at the outer ends. 30 indicates the brazed connections. A metal sleeve 31, which forms part of the evacuated envelope, is sealed hermetically at a brazed joint 32 to the top of the ceramic stem.
In this kind of conventional structure, the molybdenum cathode support rods have to be fairly long, as they are sealed hermetically at the bottom end of the stem and extend outside it. This makes these parts expensive, and in addition it is not easy to obtain by this means sufficiently rigid support for the cathode. Further, it is difficult to achieve a hermetic seal between molybdenum and Kovar (trade name) (Fe-Ni-Co alloy) and since the hermetic joint is subjected to high temperatures because of heat conducted from the cathode, it is difficult to ensure a high degree of reliability for this seal.
A different structure, that shown in FIG. 18, has been proposed in, for example, disclosed Japanese Patent Application Laid-open No. 56-132747. In this structure, hermetic sealing of the ceramic stem 27 and the cathode support rods 25, 26 is obtained by hermetic brazing using sealing rings 33 on the cathode side, i.e., on the side facing the evacuated region, of the stem. In this case, the provision of a step between the brazed joint 32 of the stem and the metal sleeve and the brazed joint 30 of the stem and the cathode support rods enhances withstand-voltage performance between the two. But this structure too has disadvantages: here again the molybdenum cathode support rods have to be long, so that the cost of these parts remains high, and overheating of the brazed joints of the cathode support rods is still likely to impair the seal. There is the additional disadvantage in both types of structure that since the hermetic brazed connections of the ceramic stem to the cathode support rods and to the metal sleeve are displaced relative to each other in the axial direction of the tube, the process of forming the metallized layer for these brazed joints is complicated.